The embodiments herein relate to propellers and, more particularly, to propeller blades formed having an inner spar and an outer spar, as well as methods associated therewith.
Modern propeller blades typically include root portions which extend into the hub arm of the hub of the propeller system and which are secured to and rotatable relative to the hub arm via a retention assembly. Typically the retention assembly includes one or a plurality of ball bearing assemblies which permit the rotation of the blade in the hub arm for accomplishing pitch change of the blade for altering the speed of the propeller and accordingly, the aircraft.
The blades are typically formed by surrounding a foam spar core with a resin impregnated fabric. Leading and trailing edges of the blade are then formed over the fabric and surrounded by, for example, a Kevlar sock. Such blades are light and effective for their intended purposes.
The spar foam is not typically considered to be a significant structural component. Rather, it is viewed as a component to provide a mandrel for over-braiding the carbon spar, and to provide a secondary means of providing shear transfer between the face and camber sides of the blade. The primary shear transfer is provided by the leading edge and trailing edge of the carbon spar. It has been observed for large propeller blades that considerable thermal stresses can occur in the spar foam during blade manufacturing and during its service life. These thermal stresses, combined with those due to aerodynamic and inertial loads, may result in undesirable structural consequences.